Polypropylene resins are widely used because of their excellent rigidity and impact resistance. In particular, molded products obtained by molding, such as injection molding, of a polypropylene resin composition are now used for automotive materials. These molded products are required to have excellent appearance, such as sense of togetherness with an automotive body, upscale image and high grade design, as well as mechanical characteristics such as rigidity and impact resistance. Additionally, the molded products are required to be produced with high efficiency to attain reduction of costs.
On the other hand, an attempt has been made to improve the physical properties of molded products and production efficiency by compounding a previously produced master batch composition containing additives such as a pigment for coloring, a softening agent, a filler and a mold releasing agent, in a relatively high concentration in a polypropylene resin as a material to be molded and by producing the molded products using the compounded resin.
With regard to the above method, a method is known in which the same polypropylene resin as that for the material to be molded is used as a carrier resin of the master batch composition. Since the same polypropylene resins show good compatibility therebetween, the master batch composition is considered to be highly dispersed in the polypropylene resin and to improve the appearance. With the above method, however, since the polypropylene resin used for injection molding has a high molecular weight and, therefore, a high melt viscosity, a problem occurs that kneading efficiency and shapability become poor when the content of the pigment is about 30% by weight or more. Further, because of high crystallinity and high crystallization rate, it is not possible to increase the amount of the additives since, otherwise, phase separation between the additives and the polypropylene resin carrier proceeds. Therefore, the production efficiency cannot be improved. Additionally, since additives such as a lubricant, an antistatic agent, a heat stabilizer and a UV absorbing agent have a lower melt viscosity than the polypropylene resin, a drawback that kneading cannot sufficiently proceed in an extruder for producing the resin composition is caused.
An attempt is also made to use a polypropylene wax having a reduced molecular weight as a carrier of a master batch. The above-described problem with respect to the melt viscosity is overcome by this method. However, because of the high crystallinity and, therefore, high melting point, a blowing agent, a modifier, a crosslinking agent, etc. are unavoidably fused and kneaded at a high temperature during the production of the master batch composition. As a result, there is a drawback that these additives undergo chemical changes to cause foaming, crosslinking, decomposition and grafting. In particular, when additives such as a flame retardant, a blowing agent, a deodorant, an antibacterial/antifungal agent are subjected to a high temperature during the melt processing of the polyolefin resin, the chemicals and additives evaporate or sublime so that a drawback that the amounts of the effective components are considerably reduced is unavoidably caused. Furthermore, because of the high crystallization rate, a problem that high density filling of these additives cannot be made is not overcome.
There is proposed a method in which an atactic polypropylene is used as a carrier resin with a low molecular weight and a low melting point for a master batch. In this case, a pigment can be dispersed in a high concentration in the master batch composition and, therefore, the costs for a coloring process can be reduced. However, while the atactic polypropylene is a by-product in the preparation of polypropylene and has a merit that it is available at a low price, residues such as a catalyst and a solvent are contained therein in a large amount. Thus, there are problems because generation of odors and coloration are caused at the time the master batch composition is prepared and because the color tone of molded products obtained from the master batch composition subtly varies to cause difficulty in color matching the molded products. Further, because the atactic polypropylene is a by-product, the physical properties thereof such as melt viscosity greatly vary. Therefore, there is a problem that the fluidity is unstable at the time of kneading and that the process efficiency is deteriorated. There are proposed a method for producing an antistatic master batch (see, for example, Patent Document 1) and a method for producing a coloring or modifying master batch (see, for example, Patent Documents 2 and 3) using such an atactic polypropylene as a carrier. These methods, which use an atactic polypropylene being a by-product in the preparation of polypropylene as a raw material, have the aforementioned defects. Use of an amorphous polyolefin random copolymer, produced as a desired product rather than as a by product, as a carrier resin for a master batch is also proposed (see, for example, Patent Documents 4 and 5). The copolymer, which is obtained using a heterogeneous catalyst, however, has wide molecular weight distribution and compositional distribution. Namely, the copolymer contains crystalline, high molecular weight components having a high melting point and amorphous, low molecular weight components having a low melting point. Since the low molecular weight components act as tacky components, the copolymer causes problems that master batch pellets are adhered to each other and molded product obtained are susceptible to blocking. In the above proposals, it is described that the use of an anti-block agent is preferred when there is tackiness. When molded products are tacky, releasability thereof from molds at the time of injection molding is reduced to cause reduction of the process efficiency and deterioration of appearance.
Further, a colored resin composition using a polypropylene wax obtained with a metallocene catalyst as a dispersing agent is proposed (see, for example, Patent Documents 6 and 7). The wax, however, has a melting point of 90° C. at minimum. A carrier for a master batch having a still lower melting point is desired for the purpose of prevention of deterioration and reaction of additives used. The wax used in the above proposal is one having a reduced melting point by copolymerizing with ethylene, etc. In order to reduce the melting point, it is necessary that the copolymer should have a high ethylene content. Thus, a problem that the compatibility with a polypropylene resin is lowered is caused.
[Patent Document 1] Japanese Unexamined Patent Application Publication No. S62-62836
[Patent Document 2] Japanese Unexamined Patent Application Publication No. S52-126449
[Patent Document 3] Japanese Unexamined Patent Application Publication No. S53-67750
[Patent Document 4] Japanese Unexamined Patent Application Publication No. H01-261440
[Patent Document 5] Japanese Unexamined Patent Application Publication No. H07-82424
[Patent Document 6] Japanese Unexamined Patent Application Publication No. 2003-183447
[Patent Document 7] Japanese Unexamined Patent Application Publication No. 2003-525340